Marine sediment composition

The failure to identify the necessary authigenic silicate phases in sufficient quantities in marine sediments has led oceanographers to consider different approaches. The current models for seawater composition emphasize the dominant role played by the balance between the various inputs and outputs from the ocean. Mass balance calculations have become more important than solubility relationships in explaining oceanic chemistry. The difference between the equilibrium and mass balance points of view is not just a matter of mathematical and chemical formalism. In the equilibrium case, one would expect a very constant composition of the ocean and its sediments over geological time. In the other case, historical variations in the rates of input and removal should be reflected by changes in ocean composition and may be preserved in the sedimentary record. Models that emphasize the role of kinetic and material balance considerations are called kinetic models of seawater. This reasoning was pulled together by Broecker (1971) in a paper called "A kinetic model for the chemical composition of sea water."... 

Records of past environmental change are preserved in a broad range of Earth materials. Past environments are inferred from "proxy" records, meaning measurements of physical and chemical parameters of marine and terrestrial sediment, polar ice, and other materials that were in some way influenced by their environment during accumulation. Examples of proxy records are the distribution of glacial deposits, the isotopic composition of terrestrial and marine sediments and ice, the abundance and species composition of plant and animal fossils, and the width of tree rings. 

Chassery et al. [97] studied the 87Sr/86Br composition in marine sediments, observing excellent agreement between results obtained by ICP-MS and thermal ionisation mass spectrometry. Low level a-spectrometry with lithium drifted germanium detectors has been used to determine 90strontium in seawater [59]. 

The haptophyte microalga Emiliania huxleyi produces biomarkers in the form of long-chain (C37, C38, and C39) alkenones (Brassell, 1993). Alkenones are well preserved in marine sediments and their molecular distributions and isotopic composition have been used to infer paleo-temperatures (Brassell, 1993) and pC022 values (Jasper et ak, 1994), respectively. Unsaturation patterns in the alkenone series are related to the growth temperature of the haptophyte algae that produce these compounds (Brassell et ak, 1986 Prahl and Wakeham, 1987), and hold great promise as indicators of absolute ocean paleotemperature. 

Gelinas, Y., J.A. Baldock, and J.I. Hedges. 2001a. Organic carbon composition of marine sediments Effect of oxygen exposure on oil generation potential. Science 294 (5540) 145-148. 

A significant amount of seawater is trapped in the open spaces that exist between the particles in marine sediments. This fluid is termed pore water or interstitial water. Marine sediments are the site of many chemical reactions, such as sulfate reduction, as well as mineral precipitation and dissolution. These sedimentary reactions can alter the major ion ratios. As a result, the chemical composition of pore water is usually quite different from that of seawater. The chemistry of marine sediments is the subject of Part 111. 

The chemical composition of seawater is largely regulated by biogeochemical processes that cause dissolved materials to be converted into solid forms. These solids are then deposited on the seafloor, making the sediments a very important reservoir in the crustal-ocean-atmosphere factory. Marine sediments are also important because they contain our only record of past conditions in the ocean. 

Marine sediments are composed of unconsolidated particles that blanket the bedrock of the seafloor. They vary greatly in chemical composition, mineralogy, particle size, origin, 327... 

Each of these solid phases can be described in terms of their mineralogy. This classification scheme is based on crystal structure and chemical composition. The most common minerals found in marine sediments are listed in Table 13.2. Most are silicates in which Si and O form a repeating tetrahedral base unit. Other minerals common to marine sediments are carbonates, sulfates, and oxyhydroxides. Less common are the hydrogenous minerals as they form only in restricted settings. These include the evap-orite minerals (halides, borates, and sulfates), hydrothermal minerals (sulfides, oxides, and native elements, such as gold), and phosphorites. 

The unique chemical composition of cosmogenous debris has provided some insight into why approximately 70% of the species of organisms on Earth were driven extinct over a relatively short time interval approximately 66 million years ago. Evidence for this mass extinction has been observed in marine sediments throughout all the ocean basins. In a contemporaneous layer deposited at the end of the Cretaceous period, the hard parts of many species of marine plankton abruptly vanished from the sedimentary record. This sedimentary layer is also characterized by a large enrichment in the rare element iridium. 

Table 18.1 Average Compositions of the Earth s Upper Continental Crust, Shale, Iron-Manganese Oxides, Phosphorite, and Various Types of Marine Sediments (All in Units of ppm. Unless Noted otherwise), along with Seawater and a Hydrothermal Vent Solution from the East Pacific Rise (both in Units of 10 g L ). 

Lithium is a conservative element in the ocean with a residence time of abont one million year. Its isotope composition is maintained by inputs of dissolved Li from rivers (average 5 Li + 23%c, Huh et al. 1998) and high-temperature hydrothermal fluids at ocean ridges at one hand and low temperature removal of Li into oceanic basalts and marine sediments at the other. Any variance in these sources and sinks thus should cause secular variations in the isotope composition of oceanic Li. And indeed in a first attempt Hoefs and Sywall (1997) interpreted Li isotope variations in well preserved carbonate shells as indicating secular variations of the oceanic Li-cycle. 

T1 isotope ratios might be also used as a tracer in mantle geochemistry (Nielsen et al. 2006 2007). Since most geochemical reservoirs except Fe-Mn marine sediments and low temperature seawater altered basalts are more or less invariant in T1 isotope composition, admixing af small amounts of either of these two components into the mantle should induce small T1 isotope fractionations in mantle derived rocks. And indeed, evidence for the presence of Fe-Mn sediments in the mantle underneath Hawaii was presented by Nielsen et al. (2006). 

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